Three astronomers will share the Nobel prize in physics, for their finding that the universe's post-Big Bang expansion is neither slowing nor retreating, but is speeding up.

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Nobel Prizes winner for physics Saul Perlmutter smiles as he poses with his daughter's telescope at his home in Berkeley, Calif., Tuesday, after hearing he had won.

Don Blake/Reuters

Nobel Prize 2011 co-winner in Physics Prof. Adam Riess is congratulated by a student (r.) at Johns Hopkins University in Baltimore, on Tuesday.

Adrian Schmidt/AP

Prof. Brian Schmidt poses for a photograph on Tuesday, at his home in Canberra, Australia, shortly after it was announced he is one of three US-born scientists to win the Nobel Prize in physics for discovering that the universe is expanding at an accelerating pace.

The award, announced Tuesday morning in Stockholm, recognizes work that represents "the biggest shakeup in the physical sciences in the last 30 years," says Phillip Schewe, a physicist and spokesman for the Joint Quantum Institute at the University of Maryland.

"It's quite a lesson in scientific humility," he says, noting that this one discovery upended the way cosmologists viewed a universe they thought they were coming to know quite well.

Even 15 years ago, "everyone agreed that the universe was expanding," he explains. Evidence for that expansion came via observations dating to the 1920s that no matter where astronomers turned their telescopes, galaxies were receding. The more distant the galaxy, the faster the pace.

Like Magic Marker dots on an expanding balloon, galaxies were being carried along by the expansion of space-time itself.

Early on, this led to the idea that the universe began in an enormous release of energy, dubbed the Big Bang. Over the years, the idea has been refined, but the general outline has been upheld by increasingly sophisticated observations, which put the event at some 13.8 billion years ago.

As scientists filled in the picture of the universe's beginning, however, its future was still open to debate, which centered on two possible paths.

If the universe held just the right density of matter and energy, the combined gravity of everything in the cosmos would allow the universe to expand forever, but at pace that would continually slow, though it would never reach zero. The other view posited that gravity ultimately would win out and throw the expansion into reverse. The universe would contract until everything collided in a cataclysmic "Big Crunch."

For a variety of reasons, most cosmologists tended to back the idea of an endless slowdown in the expansion rate. An accelerating universe was not on the cosmological radar screen.

Two teams tried to resolve the question by observing the light from exploding stars, known as supernovae, appearing at the farthest distances their telescopes would allow them to observe. Dr. Perlmutter led one team. Dr. Schmidt led the other, which included Dr. Riess.

One type of supernova in particular, known as a type 1A supernova, shows a common peak brightness wherever it pops up. By studying the spectra and the brightness of some 50 type 1A supernovae at enormous distances, the two teams independently found that the supernovae were dimmer than should be the case if the universe's expansion rate were slowing. After crunching the numbers, the data showed that the expansion was speeding up.

The teams published their results in astronomy journals in 1998 and 1999.

What is driving the expansion? That's the question astrophysicists are now trying to answer.

For many scientists, the new-found force harkens back to a concept called the cosmological constant. In essence, it was a fudge factor Albert Einstein used to make his preference for a steady-state universe hold up in the face of predictions from his general theory of relativity that the universe ultimately would contract, thanks to gravity.

The cosmological constant was a kind of antigravity that pushed out against gravity's inward tug. Later, when observations showed that the cosmos was expanding, he dubbed the cosmological constant his biggest mistake.

Detailed measurements of the background hiss from the Big Bang – itself a source for two Nobel-winning discoveries – show that what researchers dub "dark energy" represents roughly 75 percent of all the matter and energy in the universe. It pervades the universe, but its effects begin to show up only as the gravitational attraction between objects in an expanding universe weakens sufficiently.

"It's as if we found 700 new chemical elements that we didn't know about, but they were all around us all the time. It's that kind of revelation," Dr. Schewe says.

The discovery also is one more lesson in humility for humanity, he suggests. The particles that make up matter that physicists and astronomers can detect and measure account for only about 4 to 5 percent of all the matter and energy in the universe.

"The matter that we see, that we've studied so painstakingly all these years, it just a tiny, tiny fraction of the stuff that makes up the universe," Schewe says.

As for the future of the universe itself? Either the ultimate get-away-from-it-all experience, or a cold, dark, lonely one.

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